1. Field of the Invention
This invention relates to a plug removal apparatus and, more particularly, to a remotely operatable apparatus for removing plugs from plugged heat exchanger tubes.
2. Description of the Prior Art
In tube type heat exchangers, a first fluid flows through the tubes of the heat exchanger while a second fluid surrounds the outside of the tubes such that heat exchange occurs between the two fluids. Occasionally, one of the tube becomes defective and a leak occurs therein which allows the two fluids to mix. When a leak occurs, it is necessary to plug the tube so that the fluid does not flow through the tube thereby preventing leakage from the tube.
Nuclear power plants, typically pressurized water reactor power plants, include tube type heat exchangers commonly referred to as steam generators.
When a defect occurs in the tubes of the nuclear steam generator that allows the reactor coolant in the tubes to enter the feed water outside of the tubes, a more significant problem arises. Namely, the leakage creates a radioactive contamination problem. Since the coolant in the tubes of a nuclear steam generator is generally radioactive, it is important that the collant be prevented from leaking from the tubes and contaminating the fluid surrounding the tubes. Therefore, when a leak occurs in a nuclear steam generator heat exchange tube, the heat exchange tube is plugged so that the collant is not permitted to flow through the tube. This prevents contamination of the fluid surrounding the tubes.
One device used to plug heat exchange tubes is generally referred to as a mechanical plug. The mechanical plug comprises a cylindrical member closed at one end and having a tapered inner bore. A cone member is disposed in the cylindrical member such that when the cone member is drawn along the longitudinal axis of the cylindrical member, the cylindrical member is forced into close contact with the heat exchange tube due to the relative tapers of the inner surface of the cylindrical member and the cone member. This type of plug has the characteristic of being able to be removed without being drilled out. In order to remove the mechanical plug, it is, thus, first desirable to relieve the internal pressure on the plug by moving the cone member to a position in the plug in which the tapered diameters of the plug and cone member are not in contact. Next, with the pressure on the plug relieved, the plug may then be pulled from the heat exchange tube.
Of course, the plugging of heat exchange tubes occurs when the plugged tubes have become out of service. An increased number of plugged tubes results in a decreased heat exchange capacity of the heat exchanger. Therefore, it becomes necessary to remove a plug so that the plug or the associated tube can be replaced or repaired.
Such a plug removal operation is normally performed in a somewhat radioactive environment, since such plugs are usually installed in nuclear steam generators.
The operation in the radioactive environment should preferably be accomplished within a short time from the standpoint of preserving a worker's health (to meet the limitation of radiation exposure), otherwise, a long-time operation requires a number of workers.
Published European Patent No. 35911 (Japanese Patent Laid-Open No. 56-146682, U.S. Pat. No. 4,366,617) discloses an apparatus suited to achieve the pull-out and removal operation of plugs within a short time.
This prior apparatus employs a pull-out rod threadedly engageable with an open end inner surface of a plug and a fluid actuator for pushing a cone member and pulling out the plug from a heat exchange tube. These components are assembled into a single unit and can shorten the removal operation as compared with the case of using simple hand tools. However, even if this prior apparatus is used, a worker has to stand in the radioactive environment over the course of the entire operation of coupling (threadedly engaging) the pull-out rod with the plug and perform such operation. Hence, the effect of reducing the amount of radiation exposure is not enhanced.
One conventional plug removal apparatus practically used in the plug removal operation is shown in FIG. 1. Reference symbol a designates a tube plate, and b designates a heat exchange tube (provided in a hole boared in the tube plate a). The tube plate a has a number of heat exchange tubes b arranged in regular order and attached at each end thereto.
Reference symbol c designates a plug for blocking up an end portion of the heat exchange tube b, d designates a cone member for causing the plug c to contact the inner peripheral surface of the heat exchange tube b tightly through wedge action, e designates a plug removal apparatus, f designates a cone member push rod, g designates a mandrel, h designates a plug pull-out cylinder, i designates a plug pull-out piston, j designates a cylinder for moving the cone member push rod, k designates a piston for moving the cone member push rod, l designates a handle for turning the cone member push rod, and m designates a gauge indicative of the extent of travel of the cone member.
A worker brings the plug removal apparatus e into a water room of a steam generator, positions the plug removal apparatus e directly under the plug c, and turns the mandrel turning handle l by hand to cause the threaded point of the mandrel g to engage threadedly with a threaded portion c.sub.1 provided on the inner peripheral surface of the plug c.
Then, the inside of the cylinder j for moving the cone member push rod is pressurized to move both the piston k for moving the cone member push rod and the cone member push rod f so that the cone member d is pushed into the inner part of the plug c. Because the cone member d maintaining the plug c tightly in contact with the inner peripheral surface of the heat exchange tube b through wedge action is pushed into the inner part of the plug c, it becomes possible to pull out the plug c from inside the heat exchange tube b.
At this moment, the worker confirms through the gauge m indicative of the extent of travel of the cone member as to whether or not the cone member d has been pushed. If the cone member d has completely been pushed, the inside of the plug pull-out cylinder h is pressurized to move the plug pull-out piston i (to move the mandrel g), thereby pulling the plug c out. The plug c pulled out, together with the plug removal apparatus e, is brought out from the water room by the worker, detached from the mandrel f, and thrown away.
As described above, the cylinder operation carried out comprises the two steps of pressurizing the inside of the cylinder j for moving up the cone member push rod and pressurizing the plug pull-out cylinder h,and all steps are performed manually.
That is, when the foregoing conventional apparatus is used the worker must enter the water room under the radioactive environment, and hold and control the apparatus while working. Hence, the same problem as of the prior apparatus still remains.
Furthermore, both apparatuses described above have the problem of making it impossible to drive in the mandrel g at the time of pull-out because of collapse, misalignment, and the like of the threaded portion of the plug c that would occur when the plug c is attached to the heat exchange tube b.